The bicycle racks at Union Station in Washinton DC have become unsafe to lock a bicycle to, as bike thieves have removed many of the nuts holding the U-shaped racks to the concrete. In addition, many of them now have ordinary hex nuts, not the original security nuts holding them down. Anyone with a wrench can remove the hex nuts, and obviously someone is able to remove the security nuts as well. The steel bases on each end of the U sections are small enough to pass through a U-lock.
Some of the U-racks have no nuts holding them down at all and are obvious, but others are subtle, with one hex nut on each end. These are easily mistaken for "secure" and obviously bikes are being stolen or nobody would be bothering to remove the nuts.
If you must use these bike racks, first find one with at least one security nut on each end, and nuts on all four studs. Then, remove the bike's front wheel and lock it alongside the rear wheel, with the lock passing through both wheels and the frame. A thief may still be able to get the rack out of the lock by removing all four nuts, but if he does he still cannot ride the bike and it is very clumsy to carry with both wheels locked to the frame in the rear. He may also find that the space in the lock is now so full of frame and wheels that the base of the rack no longer fits through the lock. If you have a lock small enough that the bases of the racks can't pass through it at all, use it. Better idea: forget the racks entirely and lock to a signpost until this is fixed.
This sort of thing could happen anywhere this kind of oudoor bike rack is popular, so when using these anywhere, always check that all four nuts are in use and that they are security nuts, or that your lock is small enough that the feet of the rack can't pass through it.
Note: we don't need more cops to fix this, we need more mechanics. We need the right nuts on the bike racks and maybe a bit of weld or hammering over the tops of the studs so nobody can get them off with hand tools.